Method of making an alumina-silicate oxynitride and cubic boron nitride ceramic composite

ABSTRACT

A method for producing a composite of cubic boron nitride dispersed in a SiAlON ceramic. This method involves mixing silicon nitride nanoparticles, aluminum nitride nanoparticles, silica nanoparticles, calcium oxide nanoparticles, and cubic boron nitride microparticles to produce a mixture. The cubic boron nitride may be coated with nickel. The mixture is sintered to produce the composite, and this sintering may involve spark plasma sintering and/or sintering at a relatively low temperature. The composite may comprise a mixture of Ca-α-SiAlON and β-SiAlON ceramic reinforced by boron nitride in either or both cubic and hexagonal phases.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from U.S. ProvisionalApplication No. 62/430,578 filed on Dec. 6, 2016, which is incorporatedherein by reference in its entirety.

STATEMENT OF ACKNOWLEDGEMENT

The authors wish to acknowledge King Abdul-Aziz City for Science andTechnology (KACST) represented by the science and technology unit inKing Fahd University for Petroleum and Minerals (KFUPM) for funding thiswork through the National Science, Technology and Innovation Plan(NSTIP) with a project No. 13-NAN1700-04.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a method for producing a composite ofcubic boron nitride (cBN) dispersed in a SiAlON ceramic.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Ceramics serve a traditional role as durable materials that are able towithstand extreme temperatures and pressures. One of the ceramics thatexhibits outstanding thermo-mechanical resilience is silicon nitride[Riley, F. L. J. Am. Ceram. Soc 83 (2000) 245-65—incorporated herein byreference in its entirety]. However, the synthesis of fully compact anddensified silicon nitride materials is challenging given the strongcovalent character of its chemical bonds, which require excessively hightemperatures to overcome [Hampshire, S. “The Role of Additives in thePressureless Sintering of Nitrogen Ceramics for Engine Applications.”Metals Forum. Pergamon Press 7 (1984) 162-70 and Hampshire, S. MaterialsScience Forum Trans Tech Publications 606 (2009) 27-41—each incorporatedherein by reference in its entirety]. However, certain metal oxideadditives to silicon nitride, combined with synthesis at hightemperatures as well as sufficiently long sintering periods, have led tothe development of fully compact materials at much lower temperatures[Riley, F. L. J. Am. Ceram. Soc 83 (2000) 245-65; Hampshire, S. “TheRole of Additives in the Pressureless Sintering of Nitrogen Ceramics forEngine Applications.” Metals Forum. Pergamon Press 7 (1984) 162-70;Hampshire, S. Materials Science Forum Trans Tech Publications 606 (2009)27-41; and Hampshire, S. et al. “Grain Boundary Glasses in SiliconNitride: A Review of Chemistry, Properties and Crystallisation.” J. Eur.Ceram. Soc 32 (2012) 1925-32—each incorporated herein by reference inits entirety]. An obvious outcome of this additive modification was thedevelopment of sialon materials, which have an additive-controlledstructure-property relationship [Jack, K. H. et al. Nature 238 (1972)28-9; Oyama, Y. et al. Jpn. J. Appl. Phys. 10 (1971) 1637; andHampshire, S, et al. Nature 274 (1978) 880-2—each incorporated herein byreference in its entirety].

Although rare-earth metal oxides have been employed as stabilizingadditives in the sintering of sialons for decades, calcium oxide latelyhas become a favorable additive due to its higher solubility andstability [Herrmann, M. et al. J. Eur. Ceram. Soc 32 (2012) 1313-9;Menke, Y. et al., “Effect of Rare-Earth Cations on Properties of SialonGlasses.” J. Non-Cryst. Solids 276 (2000) 145-50; Bandyopadhyay, S. etal. Ceram. Int. 25 (1999): 207-13; Hakeem, A. S. et al. J. Eur. Ceram.Soc 27 (2007) 4773-81; Wang, P. L. et al. Mater. Lett. 38 (1999) 178-85;and Wang, P. L. et al. J. Eur. Ceram. Soc 20 (2000) 1333-7—eachincorporated herein by reference in its entirety]. Moreover, the lowcost and high availability of Ca-based compounds has been an additionaladvantage for their use as sintering aids [Van, R. et al. Ceram. Int. 27(2001) 461-6—incorporated herein by reference in its entirety].

While the use of metal oxide additives has enabled the sintering ofsilicon nitride ceramics at less extreme temperatures, the advent ofspark plasma sintering (SPS) has additionally provided a fastersynthesis route than traditional sintering methods such as hot pressingand hot isostatic pressing [Belmonte, M. et al. J. Eur. Ceram. Soc 30(2010) 2937-46—incorporated herein by reference in its entirety]. SPS isa consolidation technique that has gained attention for the synthesis ofceramic materials due to its higher heating rate, shorter synthesisduration, and novel pulsed-current based heating [Liu. L. et at J. Eur.Ceram. Soc 30 (2010) 2683-9 and Salmon, D. et al. J Eur. Ceram. Soc 27(2007) 2541-7—each incorporated herein by reference in its entirety].

In view of the foregoing, one objective of the present invention is toprovide a method for producing a composite of cubic boron nitride (cBN)dispersed in a sialon ceramic. Another objective is to provide adensified, e.g., fully densified, cBN-reinforced sialon compositeproduced at a sintering temperature of as low as 1500° C., which givesrise to mechanical properties having contradictory natures, i.e. highhardness along with medium to high fracture toughness.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to a methodfor producing a composite of cubic boron nitride (cBN) dispersed in aSiAlON ceramic. This method involves mixing silicon nitridenanoparticles, aluminum nitride nanoparticles, silica nanoparticles,calcium oxide nanoparticles, and cubic boron nitride (cBN)microparticles to produce a mixture, and sintering the mixture toproduce the composite.

In one embodiment, the mixing involves sonication.

In one embodiment, the mixing involves ball milling.

In one embodiment, the cBN microparticles have a largest lineardimension of 10-50 μm and are present in the mixture at a weightpercentage of 5-40 wt %, relative to a total weight of the mixture.

In a further embodiment, the mixture comprises nickel, where the nickelis located on an exterior surface of the cBN microparticles.

In a further embodiment of the above, the cBN microparticles are coatedwith nickel and comprise 20-80 wt % nickel, based on a total weight ofthe cBN microparticles.

In another further embodiment of the above, the composite has a higherfracture toughness than an otherwise identical composite sintered fromcBN microparticles that do not have nickel.

In one embodiment, the silicon nitride nanoparticles comprise α-Si₃N₄.

In one embodiment, the aluminum nitride nanoparticles have a longestlinear dimension of 30-120 nm.

In one embodiment, the aluminum nitride nanoparticles have a longestlinear dimension of 30-70 nm.

In a further embodiment, the composite has a higher fracture toughnessthan an otherwise identical composite produced from other aluminumnitride nanoparticles having a longest linear dimension of 85-500 nm.

In a further embodiment, where the aluminum nitride nanoparticles have alongest linear dimension of 30-70 nm, at least 75 wt % of the SiAlONceramic is in a β phase, relative to a total weight of the SiAlONceramic.

In a further embodiment, where the aluminum nitride nanoparticles have alongest linear dimension of 30-70 nm, the composite comprises boronnitride, and 40-95 wt % of the boron nitride relative to a total weightof the boron nitride is hexagonal boron nitride (hBN) as determined byXRD and/or Raman spectroscopy.

In one embodiment, the sintering is a spark plasma sintering process.

In one embodiment, the sintering is performed at a temperature rangingfrom 1400-1600° C.

In one embodiment, the sintering comprises heating the mixture at a rateranging from 5-600° C./min.

In one embodiment, the sintering comprises heating the mixture at a rateranging from 90-110° C./min.

In one embodiment, the sintering comprises applying a uniaxial pressureranging from 30-80 MPa to the mixture.

In one embodiment, the composite has a Vickers hardness (HV₁₀) of 8-25GPa.

In one embodiment, the composite has a fracture toughness of 5-13 MPa√m.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows X-ray diffraction (XRD) patterns of the sintered samples5-Ca-α-2, 5-Ca-α-3, and 5-Ca-α-4all of which comprise 20 μm cBNreinforcement and 100 nm AlN precursor.

FIG. 2 shows XRD patterns of samples 5-Ca-α-5 to 5-Ca-α-10, all of whichcomprise 40 μm cBN reinforcement and 50 nm AlN precursor.

FIG. 3 is a post-sintering Raman microscopy scan of the 5-Ca-α-7 sample,having 40 μm cBN particle embedded in a sialon matrix.

FIG. 4A is a field emission scanning electron microscope (FESEM)micrograph of an etched pure alpha sialon sample (5-Ca-α-1).

FIG. 4B is backscattered FESEM micrograph of the 5-Ca-α-2 compositecontaining 10 wt % of 20 μm cBN reinforcement.

FIG. 5A is a FESEM micrograph of a polished surface of the 5-Ca-α-7composite.

FIG. 5B is a FESEM micrograph of an etched surface of the 5-Ca-α-7composite.

FIG. 6 is a graph showing different sample densities as a function ofweight percentage and type of cBN reinforcement.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

The present disclosure will be better understood with reference to thefollowing definitions. As used herein, the words “a” and “an” and thelike carry the meaning of “one or more.” Within the description of thisdisclosure, where a numerical limit or range is stated, the endpointsare included unless stated otherwise. Also, all values and subrangeswithin a numerical limit or range are specifically included as ifexplicitly written out.

As used herein, “compound” is intended to refer to a chemical entity,whether as a solid, liquid, or gas, and whether in a crude mixture orisolated and purified.

As used herein, “composite” refers to a combination of two or moredistinct constituent materials into one. The individual components on anatomic level, remain separate and distinct within the finishedstructure. The materials may have different physical or chemicalproperties, that when combined, produce a material with characteristicsdifferent from the original components. In some embodiments, a compositemay have at least two constituent materials that comprise the sameempirical formula but are distinguished by different densities, crystalphases, or a lack of a crystal phase (i.e. an amorphous phase).

In addition, the present disclosure is intended to include all isotopesof atoms occurring in the present compounds and complexes. Isotopesinclude those atoms having the same atomic number but different massnumbers. By way of general example and without limitation, isotopes ofhydrogen include deuterium and tritium. Isotopes of carbon include ¹³Cand ¹⁴C. Isotopically-labeled compounds of the disclosure can generallybe prepared by conventional techniques known to those skilled in the artor by processes analogous to those described herein, using anappropriate isotopically-labeled reagent in place of the non-labeledreagent otherwise employed.

As defined here, a ceramic or a ceramic material is an inorganic, oxide,nitride, or carbide material. Ceramics are typically crystalline andnon-metallic. Some elements, such as carbon or silicon, may beconsidered ceramics. However, some ceramics may contain metal ions, suchas Ca²⁺, interspersed within its structure. Ceramic materials aregenerally resistive against compression, but yield somewhat to shearing,fracturing, and tension, as compared to metals and metal alloys.Ceramics cats typically withstand chemical erosion that occurs in othermaterials subjected to acidic or caustic environments. Ceramicsgenerally can withstand very high temperatures without degrading, suchas temperatures that range from 1,000° C. to 1,600° C. Glass is notconsidered a ceramic because of its amorphous (noncrystalline)character. A “ceramic composite,” or “ceramic matrix composite,” refersto a composite where at least one of the constituent materials is aceramic. Furthermore, “ceramic composite,” is considered synonymous with“composite ceramic.”

Sialons are ceramics based on their component elements silicon (Si),aluminum (Al), oxygen (O) and nitrogen (N), and also may be denoted as“SiAlON” or “SiAlONs.” As ceramics, sialon materials comprise a solidsolution of silicon nitride and alumina, and may be classified intomultiple phases based on different empirical formula, crystalstructures, microstructure, and thermo-mechanical properties. The mostcommon phases are α-sialon (“alpha-sialon”) and β-sialon(“beta-sialon”), however, other phases may be possible such as O-sialonor X-sialon. A single sialon material may have one phase or a mixture ofmore than one phase, such as a mixture of alpha and beta phases. Varioustypes of sialon in both alpha and beta phases have been intenselystudied during the past two decades due to their remarkable mechanicalproperties, specifically the high hardness in alpha-sialon and themodest fracture toughness in beta-sialon [Izhevskiy, V. A. et al. J.Eur. Ceram. Soc 20 (2000) 2275-95 and Ekstrom, T. et al. “SiAlONCeramics.” J. Am. Ceram. Soc 75 (1992) 259-76—each incorporated hereinby reference in its entirety].

As defined here, alpha-sialon has the formula M_(x)^(v)Si_(12-(m+n))Al_(m+n))_(n)N_(16-n) where x<2 and x=m/v, where “M” isa non-aluminum metal. As evident from the formula, m represents thenumber of Al—N bonds, and n represents the number of Al—O bonds thattogether replace m+n Si—N bonds existing as four units of Si₃N₄ withinthe alpha-sialon unit cell [Jack, K. H. et al. Nature 238 (1972) 28-9;Hampshire, S, et al. Nature 274 (1978) 880-2; Cao, G. Z. et al. Chem.Mater 3 (1991) 242-52; and Jack, K. H. J. Mater. Sci 11 (1976):1135-58—each incorporated herein by reference in its entirety]. Theother major phase of sialon ceramics is beta-sialon, which is definedhere as a sialon ceramic having a concurrent equimolar replacement of zunits of silicon and nitrogen for 7 units of aluminum and oxygen and isgenerally defined by the formula Si_(6-z)Al_(z)O_(z)N_(8-z). Here, z mayrange from more than 0 to less than 4.2. In a preferred embodiment, zmay range from 1-4, more preferably 1-3. Unlike alpha-sialon, thebeta-sialon structure does not include a non-aluminum metal.

In the formula for alpha-sialon, “M” may be a metal, other thanaluminum, and is typically a lanthanide (for example, Nd, La, and Yb) orsortie other rare earth metal. Sialons typically include the identity ofM in their name as in Yb-sialon, Yb-α-sialon, or Yb-α-SiAlON to denotethat the ceramic comprises Yb as “M,” and a sialon having anincorporated metal may be considered “toughened.” The metal may be addedas a sintering aid to improve ceramic densification using a lower energyinput for sintering (i.e. lower sintering temperature and/or a shortersintering time). However, lanthanides have large ionic radii and poorlyoccupy the interstitial sites of Si₃N₄, producing ceramics with crystaldefects. In addition, introducing lanthanides or rare-earth metals intoceramics is cost-prohibitive. To address these issues, alkaline earthmetals and/or oxides of alkaline earth metals may be used as sinteringaids. Among these candidates, calcite oxide is promising because thecalcium atom can reside in an α-SiAlON structure without distorting thecrystal structure [Wang, P. L. et al. Mater. Lett. 38 (1999) 178-85 andWang, P. L. et al. J. Eur. Ceram. Soc 20 (2000) 1333-7—each incorporatedherein by reference in its entirety]. In other embodiments, “M” may beBa, Eu, Sr, Ra, Mg, or Be, and may also be added to the mixture as anoxide. In alternative embodiments, an α-sialon may have more than onetype of non-aluminum metal, for instance, Yb and Ca, having a Yb to Camass ratio of 1:1,000-1,000:1, preferably 1:100-100:1, more preferably1:10-10:1.

With certain precursor sizes or conditions (such as heat and/orpressure), or in the presence of certain additives, sialons maytransform from one phase to another, or may transform from a singlephase to having two or more phases. For example, a portion of anα-sialon may transform into a β-sialon during sintering. In thistransformation, the non-aluminum metal may migrate or leech out of theceramic by the formation of a liquid phase, and the liquid phase mayfacilitate the other changes to the chemical and structuralcompositions. However, in some embodiments, the mixture may produce aliquid phase during the sintering while resulting in only a single phaseα-sialon. In another embodiment, a sialon may transform into anon-sialon ceramic during sintering. In another embodiment, an α-sialonmay transform into an α-sialon having a formula of the type M_(x)^(v)Si_(12-(m+n))Al_(m+n)O_(n)N_(16-n), but with a different value of mand/or n. Likewise, a β-sialon may transform into a β-sialon of theformula Si_(6-z)Al_(z)O_(z)N_(8-z), but with a different value of z.

Another technique to achieve ceramic densification at a lower sinteringtemperature is to employ submicron-sized and/or nano-sized startingpowders, and to use a reinforcing particle, such as cBN or nickel coatedcBN. Such techniques will be described hereinafter.

According to a first aspect, the present disclosure relates to a methodfor producing a composite of cubic boron nitride (cBN) dispersed in aSiAlON ceramic. This method involves mixing silicon nitridenanoparticles, aluminum nitride nanoparticles, silica nanoparticles,calcium oxide nanoparticles, and cubic boron nitride (cBN)microparticles to produce a mixture, and sintering the mixture toproduce the composite.

The silicon nitride nanoparticles, aluminum nitride nanoparticles,silica nanoparticles, calcium oxide nanoparticles, and cBNmicroparticles may be spheres, spheroids, ellipsoids, flakes, cubes,prisms, or irregular shapes with carved and/or flat surfaces, unlessotherwise specified. In a preferred embodiment, the nanoparticles arespheres or substantially spherical, and the cBN microparticles arecubic, rectangular, tetragonal, or cuboid. As used herein, a diameter ofa particle, including nanoparticles and microparticles, refers to thegreatest possible distance measured from one point on the particlethrough the center of the particle to a point directly across from it. Adiameter of a flake, as used herein, refers to the greatest possibledistance measured from a first point on a perimeter of the flake throughthe center of the flake to a second point, also on the perimeter of theflake, directly across from the first point. The diameters of theparticles are described hereinafter.

In one embodiment, the silicon nitride (Si₃N₄) nanoparticles maycomprise one of α-Si₃N₄, β-Si₃N₄, γ-Si₃N₄, amorphous Si₃N₄, and mixturesthereof. In a preferred embodiment, the silicon nitride nanoparticlescomprise α-Si₃N₄. The silicon nitride nanoparticles may have a diameterof 50-500 nm, preferably 100-400 nm, more preferably 250-350 nm, thoughin some embodiments the silicon nitride nanoparticles may have may havediameters less than 50 nm or greater than 500 nm. In another embodiment,the silicon nitride particles are amorphous and have a diameter rangingfrom 1-100 nm, preferably 10-50 nm, more preferably 10-30 nm. In oneembodiment, a purity of the silicon nitride nanoparticles is more than98 wt %, preferably more than 99 wt %, more preferablymore than 99.9 wt% relative to the total weight of the silicon nitride particles.

In one embodiment, the aluminum nitride (AIN) nanoparticles have adiameter or longest linear dimension of 30-120 nm, preferably 40-110 nm,more preferably 80-105 nm, or about 100 nm. In another embodiment, thealuminum nitride nanoparticles have a diameter or longest lineardimension of 30-70 nm, preferably 40-60 nm, more preferably 45-55 nm, orabout 50 nm. However, in other embodiments, the aluminum nitridenanoparticles may have a diameter or longest linear dimension of lessthan 30 nm or greater than 120 nm, for instance, 120-500 nm. In oneembodiment, a purity of the aluminum nitride nanoparticles is more than98 wt %, preferably more than 99 wt %, more preferably more than 99.9 wt% relative to the total weight of the aluminum nitride particles.

In one embodiment, the silica (SiO₂) nanoparticles may have a diameterof 20-80 nm, preferably 25-55 nm, more preferably 35-45 nm, or about 40nm. However, in some embodiments, the silica nanoparticles may have adiameter less than 20 nm or greater than 80 nm. In one embodiment, apurity of the silica nanoparticles is more than 98 wt %, preferably morethan 99 wt %, more preferably more than 99.9 wt % relative to the totalweight of the silica particles. The silica particles may be crystallineor amorphous.

In one embodiment, the calcium oxide (CaO) nanoparticles may have adiameter of 40-300 nm, preferably 100-180 nm, more preferably 155-165nm, or about 160 nm. However, in some embodiments, the silicananoparticles may have a diameter less than 40 nm or greater than 300nm. In one embodiment, a purity of the calcium oxide nanoparticles ismore than 98 wt %, preferably more than 99 wt %, more preferably morethan 99.9 wt % relative to the total weight of the calcium oxidenanoparticles.

In one embodiment, the cBN microparticles may have a diameter or largestlinear dimension of 10-50 μm, preferably 15-30 μm, more preferably 17-23μm, or preferably 30-45 μm, more preferably 37-43 μm. However, in someembodiments, the cBN microparticles may have a diameter or largestdimension of less than 10 μm (for instance, as nanoparticles) or greaterthan 50 μm. In an alternative embodiment, microparticles of boronnitride comprising phases instead of or in addition to cubic boronnitride (cBN) may be used in the mixture. For instance, boron nitridemay be in the form of amorphous boron nitride, hexagonal boron nitride(hBN), or rhombohedral (rBN) or Wurtzeit (wBN) boron nitride allotropes.Boron nitride may also be in the form of nanotubes. In one alternativeembodiment, boron nitride in two or more of the mentioned forms may beused in the mixture.

In one embodiment, the cBN microparticles are present in the mixture ata weight percentage of 5-40 wt %, preferably 8-32 wt %, more preferably8-12 wt %, or more preferably 18-22 wt %, or more preferably 28-32 wt %,relative to a total weight of the mixture. However, in some embodiments,the cBN microparticles may be present in the mixture at a weightpercentage lower than 5 wt % or greater than 40 wt % relative to a totalweight of the mixture. In one embodiment, the mixture comprises nickel,and the nickel is located on an exterior surface of the cBNmicroparticles or entirely covering individual cBN microparticles. Thenickel may be in the form of nanoparticles having diameters of 20-500nm, preferably 50-300 nm, more preferably 80-150 nm. In one embodiment,the cBN microparticles are coated with nickel (Ni) or have attachednanoparticles of nickel, and the nickel is present at a weight percentof 20-80 wt %, preferably 30-70 wt %, more preferably 35-65 wt %relative to a total weight of the cBN microparticles. However, in someembodiments, where the nickel is located on an exterior surface of thecBN microparticles or entirely covering individual cBN microparticles,the cBN microparticles may comprise less than 20 wt % or greater than 80wt % nickel. In alternative embodiments, the cBN microparticles may becoated or in contact with different metals or compounds, for instance,Ti, TiN, Nm, Nb, Cr, V, W, Mo, and/or steel.

In one embodiment, the relative amounts of silicon nitridenanoparticles, aluminum nitride nanoparticles, silica nanoparticles, andcalcium oxide nanoparticles may be chosen to satisfy the empiricalformula Ca_(x)Si_(12-(m+n))Al_(m+n)O_(n)N_(16-n), where x<2 and x=m/2,which describes an Ca-alpha-sialon (Ca-α-sialon) ceramic.

In one embodiment, relative amounts of silicon nitride nanoparticles,aluminum nitride nanoparticles, silica nanoparticles, and calcium oxidenanoparticles may be mixed to provide a relative amount of Ca, Si, Al,O, and N having a weight percent composition of 4-7 wt % Ca, preferably5-6 wt % Ca, or about 5.4 wt % Ca; 42-49 wt % Si, preferably 43-47 wt %Si, or about 43.6 wt % Si; 8-14 wt % Al, preferably 9-13 wt % Al, orabout 12.75 wt % Al; 1-5 wt % O, preferably 1.5-4 wt % O, or about 3.2wt % O; and 33-38 wt % N, preferably 34-37 wt % N, or about 35.0 wt % N,each relative to a total weight of Ca, Si, Al, O, and N. In alternativeembodiments, other compounds comprising those elements such as metallicAl, Al₂O₃, or CaO₃ may be used to make a mixture having similar weightpercentages of components.

However, in other embodiments, AIN may be the only source of aluminum inthe mixture, meaning that other aluminum compounds such as aluminummetal and alumina are not added. Similarly, in one embodiment, CaO maybe the only source of calcium in the mixture. In another relatedembodiment, Si₃N₄ may be the only source of Si, meaning that only CaO,Si₃N₄, AIN, and Al₂O₃ may be added to the mixture, as SiO₂ is notneeded. Other combinations for the mixture may be possible, and may bedetermined by a person having ordinary skill the art. In otheralternative embodiments, a compound may be added to the mixture thatcomprises an additional element, such as carbon in the case of addingCaCO₃, and the additional element may escape or leach out during theprocess of making the composite, for instance, during the sinteringstep.

In one embodiment, the relative amounts of the nanoparticles are chosento satisfy the empirical formula Ca_(a)Si_(b)Al_(c)O_(d)N_(e), where ais 0.6-0.9, preferably 0.75-0.85; where b is 9.1-10.0, preferably9.15-9.25; where c is 2.0-2.9, preferably 2.75-2.85; where d is 0.6-1.3,preferably 1.15-1.25; and where e is 14.7-15.4, preferably 14.75-14.85.In a preferred embodiment, the relative amounts of the nanoparticles arechosen to satisfy the empirical formulaCa_(0.8)Si_(9.2)Al_(2.8)O_(1.2)N_(14.8) or an empirical formula havingapproximately similar relative amounts, which is an empirical formulafor a single phase Ca-alpha-sialon ceramic, where m=1.6 and n=1.2. Inother embodiments, a single phase Ca-alpha-sialon empirical formula maybe pursued in which m is 0-3 (with n>0), preferably 0.5-2.0, morepreferably 1.5-1.8, and n is 0-4.2 (with n>0), preferably 0.5-3.0, morepreferably 0.8-2.0, even more preferably 1.0-1.4, and in certainalternative embodiments, m may be greater than 3, and/or n may begreater than 4.2. In one embodiment, the empirical formula determinedfrom a composite may be slightly different than the empirical formula ofthe mixture before the sintering. Of a formed composite, weightpercentages or empirical formula of Ca, Si, Al, O, and N may bedetermined by atomic absorption spectroscopy, energy dispersive X-rayspectroscopy, a carrier-gas-heat-extraction method, or some othertechnique.

In one embodiment, a total mass of the mixture may be 1-100 g,preferably 2-50 g, more preferably 3-10 g, or about 5 g. Depending onuse, some embodiments may use a total mass of the mixture that isgreater than 100 g, for example, in a pilot plant or in an industrialscale chemical plant.

In one embodiment, the mixing of the mixture components (silicon nitridenanoparticles, aluminum nitride nanoparticles, silica nanoparticles,calcium oxide nanoparticles, and cBN microparticles) involvessonication. In a preferred embodiment, the powders are dispersed in asufficient amount of an organic solvent, preferably volatile at roomtemperature, to form a slurry which is sonicated for 10-60 minutes,preferably 25-35 minutes. The sonication may be applied by inserting anultrasonic probe into the slurry or by placing a container of the slurryinto a sonication bath. The sonication may be pulsed or continuous.Non-limiting examples of the organic solvent include hydrocarbons, suchas hexane, alcohols, such as ethanol, methanol, propanol, isopropanol,butanol, ketones and esters. Preferably, the solvent is an alcohol. Morepreferably, the alcohol has a melting point lower than 0° C. and aboiling point lower than 100° C. In a preferred embodiment, the alcoholis ethanol. The organic solvent may act as a viscosity modifying agent,providing a suitable viscosity for handling the slurry and accomplishingthe mixing. In addition, the solvent may have a viscosity ranging from0.5-2 cP, preferably 0.5-1.5 cP, more preferably 0.5-1.2 cP. Any amountof liquid that accomplishes the mixing is acceptable. Preferably, thesolids content is between 15-50 vol %, preferably 15-35 vol %, morepreferably 20-30 vol % of the total volume of the slurry. Below thislimit, mixing may be ineffective or separation by settling may occur,although a solid content below this limit may still be used depending onthe particle size, solvent, and mixing procedure. Above the limit, insome instances, the viscosity may be too high and mixing andde-agglomeration may not be effective. The volatile organic solvent mayevaporate during sonication, leaving no residue. Preferably, after thesonication, the slurry is heated to 60-120° C., preferably 70-90° C. for1-24 hours, preferably 8-24 hours, more preferably 10-24 hours to removethe solvent completely. In one embodiment, the sonication may break upparticles and decrease particle sizes by 10-90%, preferably 20-70%,relative to a particle size before the sonicating. However, in someembodiments, the particle size distribution may not change substantiallybefore and after the sonicating.

In one embodiment, the mixing of the mixture components involves ballmilling, or high energy ball milling. The mixture components may bemilled with a miller, such as a planetary miller, an attrition mill, avibratory mill or a high energy miller. Non-limiting examples of millingmedia (i.e. bowl and balls) include zirconium dioxide, tungsten carbide,silicon nitride, and alumina. In one embodiment, silicon nitride millingmedia is employed to minimize contamination of the powder mixture. Theballs used for milling may have a diameter of 200-1,000 μm, preferably300-900 μm, more preferably 400-800 μm, even more preferably 600-650 μm,though balls with diameters smaller than 200 μm or greater than 1,000 μmmay be used. In one embodiment, a weight ratio of the balls to thepowder mixture ranges from 4:1 to 35:1, preferably from 5:1 to 30:1,more preferably from 10:1 to 25:1. A process control agent, such asstearic acid or ethanol, may be added to the powder mixture to ensurethe powder mixture does not cake. Preferably ethanol is used. An amountof the process control agent ranges from more than 0 wt % to 2 wt %,preferably 0.5-1.5 wt %, more preferably 0.5-1 wt % of the weight of thepowder mixture. In a preferred embodiment, no process control agent isemployed. In one embodiment, the milling is performed in an inertatmosphere, preferably provided by argon gas, though in anotherembodiment, the milling may be performed in air. The powder mixture maybe milled for up to 10 hours, or up to 5 hours, or up to 2 hours,preferably for 10-90 minutes, preferably for 45-75 minutes. Ahigh-energy ball milling apparatus may use a rotation rate of2,000-10,000 rpm, preferably 2,500-5,000 rpm, more preferably2,750-3,250 rpm. Preferably, the ball milling decreases the size of theparticles by 30-95%, preferably 40-90%, more preferably 60-90% relativeto a size of the particles before the ball milling. In one embodiment,ball milling in air may lead to slight chemical changes of the mixturecomponents. For instance, a portion of ball milled CaO may react withair to form CaCO₃.

In one embodiment, the mixture components may be mixed incrementally,for instance, the nanoparticles may be mixed first by sonication, andthen cBN microparticles may be added and then the mixture furthersonicated, in order to not substantially decrease the size of the cBNmicroparticles. Alternatively, the nanoparticles may be ball milled, andthen cBN added and mixed by sonication. In an alternative embodiment,the nanoparticles and cBN microparticles may be mixed by a mortar andpestle, or by other means, such as a blade grinder or burr grinder.Changes in particle sizes before and after the mixing may be measured byfield emission scanning electron microscopy (FESEM), dynamic lightscattering, or other techniques.

The sintering process may be hot pressing, hot isostatic pressure,pressureless sintering, or spark plasma sintering. In one embodiment,the sintering is a spark plasma sintering process. Spark plasmasintering may be preferred over the other sintering processes becausespark plasma sintering densifies the compacted powders more quickly andat relatively low temperatures. Thus, the formation of secondary phaseswith different properties may be controlled. For instance, changes inthe sintering conditions (pressure, time, temperature, heating rate,cooling rate, starting particle size) may affect the weight ratio of theα-sialon and β-sialon phases that result in the ceramic composite.

For the sintering process, the mixture may be transferred to a die witha diameter of 10-50 mm, preferably 15-35 mm, mare preferably 15-25 mm.The die may comprise graphite. The powder mixture may be compacted intothe die at ambient temperature, or while heating or sintering. Thecompacted powder mixture may be in the form of a disc having a similardiameter as the graphite die, and a thickness of 1-40 mm, preferably2-15 mm, more preferably 3-10 mm. However, in other embodiments, thecompacted powder mixture may be in other forms, such as a rectangularprism, depending on the shape of the die. In a preferred embodiment, auniaxial pressure is applied to the die in a direction that is normal tothe ground. In one embodiment, the sintering comprises applying auniaxial pressure to the powder mixture, where the uniaxial pressure maybe 30-80 MPa, preferably 35-70 MPa, more preferably 40-60 MPa, even morepreferably 45-55 MPa. However, in other embodiments, pressures lowerthan 30 MPa or greater than 80 MPa may be used successfully.

In one embodiment, the sintering comprises heating the mixture at a rateranging from 5-600° C./min, preferably 120-500° C./min, more preferably200-400° C./min. In another embodiment, the sintering comprises heatingthe mixture at a rate ranging from 90-110° C./min, preferably 95-105°C./min, or about 100° C./min. However, in other embodiments, heatingrates of lower than 5° C./min or greater than 600° C./min may be usedsuccessfully. The heating may comprise one or more heating steps. In apreferred embodiment, the heating consists of only one heating step.

In one embodiment, the sintering is performed at a temperature rangingfrom 1400-1600° C., preferably 1420-1550° C., more preferably 1450-1520°C., even more preferably about 1500° C., or in one embodiment, nogreater than 1500° C.

In a spark plasma sintering process, composite starts to cool down oncethe current is switched off. The cooling of the composite may becontrolled and/or accelerated with a pre-set program. In a preferredembodiment, the composite is cooled down at a rate ranging from 1-20°C./s, preferably 1-10° C./s, more preferably 2-5° C./s. The compositemay be cooled by a flow of an inert gas, such as nitrogen or argon. Thecomposite may be cooled to a temperature ranging from 20-40° C.,preferably 20-30° C., more preferably 20-25° C. In one embodiment, thecomposite may be cooled down to 20-30° C. within 15 minutes, preferablywithin 12 minutes, even more preferably within 10 minutes. The compositemay be cleaned to remove graphite or other contaminants from the die orsintering apparatus, and the composite may be cut or polished.

As noted earlier, the first aspect of the present disclosure relates toa method of producing a composite of cubic boron nitride (cBN) dispersedin a sialon ceramic. In one embodiment, the sialon consists of only theα-sialon phase. In another embodiment, the sialon consists of only theβ-sialon phase. However, in another embodiment, the sialon ceramic ofthe composite comprises both α and β phases. For instance, a weightratio of the α phase to the β phase may be 1:1,000-1,000:1, preferably1:100-100:1, more preferably 1:50-50:1, even more preferably 1:10-10:1,though in some embodiments, a weight ratio of the α phase to the β phasemay be smaller than 1:1,000 or larger than 1,000:1. In one embodiment,the weight ratio of the phases may be estimated by comparing the peakarea ratio of the corresponding XRD patterns. In an alternativeembodiment, the conposite may comprise a sialon ceramic in a crystalphase other than α and β phases, or a part of the sialon ceramic may beamorphous. A portion, or all of the β phase may be β-Si₁₀Al₂O₂N₁₄, whichmay also be written as β-Si₅AlON₇, where z=1 as in the previouslypresented chemical formula.

In a preferred embodiment, the ceramic composite also comprises cBN,which is dispersed within the sialon ceramic. In one embodiment, the cBNmay be in the form of microparticles, which may have diameters asdescribed previously for the cBN microparticles. In another embodiment,the cBN in the composite may have nickel on the cBN or coating the cBNas previously described. However, in another embodiment, the sinteringor mixing may change the size or shape of the cBN. For instance, thediameter of the cBN particles may decrease by 2-80%, preferably 10-70%relativeto their original diameter, however, in an alternativeembodiment, the diameter may increase if two or more cBN particlescontact each other and fuse together during the sintering. Similarly,the shape or size of the nickel on a cBN particle may change during thesintering. FIG. 5A shows an FESEM image of a composite with cBNparticles (black) dispersed within.

In one embodiment, the composite comprises hBN, and this may be producedby cBN being converted to hBN during the sintering process. For ceramiccomposites containing boron nitride (BN) as a constituent material orreinforcement, the conversion of cBN to hBN in a ceramic composite maylower hardness, lower density, increase porosity, and/or promotecracking of a composite. In one embodiment, the composite only consistsof boron nitride in the form of cBN. In an alternative embodiment, thecomposite only consists of boron nitride in the form of hBN. In anotherembodiment, the ceramic comprises boron nitride in the form of both cBNand hBN. For instance, a weight ratio of cBN to hBN phase may be1:1,000-1,000:1, preferably 1:100-100:1, more preferably 1:50-50:1, evenmore preferably 1:10-10:1, though in some embodiments, a weight ratio ofcBN to hBN may be smaller than 1:1,000 or larger than 1,000:1. In oneembodiment, the weight ratio of cBN to hBN may be estimated by comparingthe peak area ratio of the corresponding XRD patterns. The presence of aphase may also be studied by Raman microscopy. In an alternativeembodiment, the composite may comprise boron nitride in a crystal phaseother than cBN and hBN, or the boron nitride may be amorphous.

In one embodiment, the composite does not have pores. However, in analternative embodiment, the composite may have micropores and/orsubmicron pores. The size of the micropores may range from 1-5 μm,preferably 1-4 μm, more preferably 1-3 μm. The size of the submicronpores may range from 50-400 nm, preferably 100-300 nm, more preferably100-250 nm. In this alternative embodiment, where the composite haspores, the porosity of the composite is at most 20%, preferably at most15%, preferably at most 5%, and more preferably at most 1%.

In one embodiment, the composite has a Vickers hardness, HV₁₀, (ASTME384 Knoop and Vickers Hardness Testing) of 8-25 GPa, preferably 12-25GPa, more preferably 20-25 GPa, or preferably 8-18 GPa, more preferably10-16 GPa. However, in some embodiments, the composite may have aVickers hardness of less than 8 GPa or greater than 25 GPa. Here, theVickers hardness is measured under a load of 98 N (10 kg).

In one embodiment, the composite has a fracture toughness of 5-13 MPa√m,preferably 6-11 MPa√m, more preferably 6-10 MPa√m, or preferably 10-13MPa√m, more preferably 11-13 MPa√m. However, in some embodiments, thecomposite may have a fracture toughness of less than 5 MPa√m or greaterthan 13 MPa√m. The unit MPa√m may also be written as MPa·m^(1/2). Thefracture toughness may be determined by the indentation method or someother technique.

In one embodiment, the composite may have a density of 2.0-4.0 g/cm³,preferably 2.5-3.8 g/cm³, more preferably 2.7-3.4 g/cm³. In oneembodiment, the composite may have a density of 3.35-3.45 g/cm³ or2.75-3.05 g/cm³. However, in some embodiments, the composite may have adensity of less than 2.0 g/cm³ or greater than 4.0 g/cm³. The densitymay be measured based on Archimedes' method with deionized water as theimmersion medium, using density determination equipment. The relativedensity of the composite may be measured with respect to a referencesintered sialon ceramic, such as an otherwise similar sintered sialonmade without cBN reinforcement, or to an otherwise similar compositemade without nickel-coated cBN reinforcement, or to an otherwise similarcomposite that is conventionally sintered without spark plasmasintering.

In one embodiment, the grain size of the composite may be studied, forinstance, by field emission scanning electron microscopy (FESEM), or,more generally scanning electron microscopy (SEM). A composite beingstudied by FESEM may first be polished or etched. Where the compositecomprises both α and β-sialon ceramic, either or both phases may appearas elongated grains, needle-like grains, flake-like grains, and/orequiaxed grains. Preferably, the β-sialon phase is in the font ofelongated grains, needle-like grains, and/or flake-like grains, and anembodiment of this is shown in FIG. 5B. The elongated grains may have aheight ranging from 1-20 μm, preferably 1-15 μm, more preferably 1-10μm, and a width ranging from 0.1-4 μm, preferably 0.5-3 μm, morepreferably 0.5-2 μm. The aspect ratio (height divided by width) of theelongated grains may range from 2-200, preferably 10-150, morepreferably 10-100. The needle-like grains may have a length ranging from1-20 μm, preferably 1-15 μm, more preferably 1-10 μm. The flake-likegrains may have a diameter of 1-15 μm, preferably 5-15 μm, morepreferably 7-10 μm. Preferably the α-sialon ceramic is in the form ofequiaxed grains. The equiaxed grains may have a diameter ranging from100-1,000 nm, preferably 200-600 nm, more preferably 300-500 nm.

In one embodiment, the ratio of constituent phases in the composite (inparticular, the weight ratio of the α-sialon to the β-sialon phase,and/or the weight ratio of cBN to hBN) may be dependent on differentparameters of the method. In an additional embodiment, other physicalcharacteristics of the composite may depend on different parameters ofthe method. Such physical characteristics include, but are not limitedto, hardness, density, grain size/shape, porosity, dispersion ofconstituent compounds, and fracture toughness. These influencing methodparameters include, but are not limited to, weight percent compositions,particle size, mixing method, type of sintering, sintering conditions(heating rate, atmosphere, pressure, time, temperature, cooling rate,sample mass, etc.), and the addition of additives.

In one embodiment, the wt % of cBN microparticles initially added to themixture before the sintering, relative to a total weight of the mixture,may influence the weight ratio of α-sialon and β-sialon phases in thecomposite. For instance, increasing the wt % of cBN from 0 to 10 wt %,from 10 to 30 wt %, or from 0 to 30 wt % may increase the β-sialon phaseby 20-95 wt %, preferably 30-92 wt %, even more preferably 50-90 wt %relative to a total mass of the sialon ceramic.

In one embodiment, the wt % of cBN microparticles initially added to themixture before the sintering, relative to a total weight of the mixture,may influence the Vickers hardness in the composite. For instance,increasing the wt % of cBN from 0 to 10 wt %, from 10 to 30 wt %, orfrom 0 to 30 wt % may increase the Vickers hardness by 1-30%, preferably2-20%, relative to an initial value of the Vickers hardness.Alternatively, increasing the wt % of cBN as described previously mayinstead decrease the Vickers hardness by 10-70%, preferably 15-65%, evenmore preferably 20-60% relative to an initial Vickers hardness.

In one embodiment, the wt % of cBN microparticles initially added to themixture before the sintering, relative to a total weight of the mixture,may influence the fracture toughness in the composite. For instance,increasing the wt % of cBN from 0 to 10 wt %, from 10 to 30 wt %, orfrom 0 to 30 wt % may increase the fracture toughness by 1-30%,preferably 2-20%, relative to an initial value of the fracturetoughness. Alternatively, increasing the wt % of cBN as describedpreviously may decrease the fracture toughness by 10-70%, preferably15-65%, even more preferably 20-60% relative to an initial fracturetoughness.

In one embodiment, the presence of nickel on an exterior surface of cBNmicroparticles initially added to the mixture before the sintering mayinfluence the fracture toughness of the sintered composite. In oneembodiment, the composite has a higher fracture toughness than anotherwise identical composite sintered from cBN microparticles that donot have nickel. In one embodiment, the presence of nickel may show anincrease of fracture toughness that is at least 5% greater, preferablyat least 10% greater, more preferably at least 20% greater relative townotherwise identical composite that does not comprise nickel. While notbeing bound to any particular explanation br theory, the compositeproduced from cBN with nickel may lead to a greater fracture toughnessdue to smoother interface between the reinforcement and the sialonmatrix.

In one embodiment, the presence of nickel on the cBN microparticlesinitially added to the mixture before the sintering may influence aweight ratio of α-sialon and β-sialon phases in the composite, ascompared to an otherwise identical composite sintered from cBNmicroparticles that do not have nickel. For instance, the weight ratioof α-sialon and β-sialon phases may decrease when nickel is added to thecBN microparticles, meaning that the nickel leads to relatively moreβ-sialon.

In one embodiment, the particle size of AlN (aluminum nitride) used inthe mixture may influence the fracture toughness, the Vickers hardness,the weight ratio of α-sialon to β-sialon phases, and the weight ratio ofcBN to hBN. The degree of these changes may be similar to what wasdescribed above for changing different properties of the cBNmicroparticles used in the mixture. In other embodiments, the particlesize of the other nanoparticle precursors, such as CaO, may similarlyinfluence the properties and phase ratios of the composite. In oneembodiment, the particle size of all nanoparticle precursors may bereduced, for instance, by ball milling, in order to produce compositeswith different properties.

In one embodiment, where the aluminum nitride nanoparticles have alongest linear dimension of 30-70 nm, preferably 40-60 nm, the compositehas a higher fracture toughness than an otherwise identical compositeproduced from other aluminum nitride nanoparticles having a longestlinear dimension of 85-500 nm, preferably 90-400 nm. In one embodiment,the fracture toughness of a composite made from the smaller aluminumnitride nanoparticles (30-70 nm diameter) may be increased 20-300%,preferably 30-200%, more preferably 35-150% relative to the fractiontoughness of an otherwise identical composite made from the largeraluminum nitride nanoparticles (85-500 nm).

In a related embodiment, where the aluminum nitride nanoparticles have alongest linear dimension of 30-70 nm, preferably 40-60 nm, the compositehas a lower Vickers hardness than an otherwise identical compositeproduced from other aluminum nitride nanoparticles having a longestlinear dimension of 85-500 nm, preferably 90-400 nm. For instance, theVickers hardness of a composite made from the smaller aluminum nitridenanoparticles may be decreased 5-80%, preferably 10-70%, more preferably15-65% relative to the Vickers hardness of an otherwise identicalcomposite made from the larger aluminum nitride nanoparticles.

In one embodiment, where the aluminum nitride nanoparticles have alongest linear dimension of 30-70 nm preferably 40-60 nm, at least 75 wt%, preferably at least 85 wt %, more preferably at least 95 wt % of theSiAlON ceramic is in a β phase, relative to a total weight of the SiAlONceramic. While not being bound to any particular explanation or theory,it may be that the smaller particle size of the AlN precursor (30-70 nmvs. 80-500 nm), and thus higher surface area, facilitated thetransformation of alpha-sialon into beta-sialon at sinteringtemperatures of 1400-1600° C., preferably 1450-1550° C. This highersurface area of AlN nanoparticles may enable a higher reactivity increating a larger amount of oxygen-rich liquid during the sintering,leading to the transformation of α-sialon to β-sialon.

In another embodiment, where the aluminum nitride nanoparticles used inthe mixture have a longest linear dimension of 30-70 nm, preferably40-60 nm; 40-95 wt %, preferably 45-93 wt %, more preferably 50-90 wt %of the boron nitride relative to a total weight of the boron nitride inthe composite is hexagonal boron nitride (hBN). This amount of the hBNphase may be determined by XRD and/or Raman spectroscopy. However, insome embodiments, lower than 40 wt % or greater than 95 wt % of theboron nitride is hBN. In one embodiment, substantially all of the boronnitride (i.e, 99.9 wt % boron nitride or greater) is hBN.

In a related embodiment, where the aluminum nitride nanoparticles usedin the mixture have a longest linear dimension of 80-500 nm, preferably90-400 nm; 0-60 wt %, preferably 0.1-30 wt %, more preferably 0.2-20 wt% of the boron nitride relative to a total weight of the boron nitridein the composite is hexagonal boron nitride (hBN).

In one embodiment, a material may comprise the composite. The materialmay be a part of an abrasive, a tool, a vehicular part, an aerospacecomponent, an engine component, a turbine component, a break-ring, anozzle, a reactor component, a high temperature refractory shape, aglass forming tool, a mold, a die, a refractory for metal forming, afurnace vent, a stack, or a fixture.

The examples below are intended to further illustrate protocols forpreparing, and characterizing the composite of cubic boron nitride (cBN)dispersed in a SiAlON ceramic, and uses thereof, and are not intended tolimit the scope of the claims.

EXAMPLE 1 Experimental Procedure

Single-phase alpha sialon and other compositions of alpha (α) and beta(β) alumina silicate oxynitride samples were synthesized, with a ratioof precursors to produce the general formula ofCa_(0.8)Si_(9.2)Al_(2.8)O_(1.2)N_(14.8). The m and n valuescorresponding to the single phase alpha sialon region were selected asm=1.6 and n=1.2.The precursors employed for synthesis includealpha-Si₃N₄ (250-350 nm, Ube Industries SN-10, Japan), AlN (SigmaAldrich, Germany). SiO₂ (40 nm, Sigma Aldrich, Germany), and CaO (160nm, Sigma Aldrich, Germany). The classification of the sialon compositesbased upon the size of precursor as well as the size and weight percentof reinforcement (cBN) employed is summarized in Table 1.

Powder precursors satisfying the aforementioned general formula ofalpha-sialon were carefully weighed to form 5 g samples. The initialpowders along with the respective weight percent of reinforcements weremixed in an ultrasonic probe sonicator for 30 min, utilizing ethanol asa sonicating medium.

Some powder precursors were ball milled. High-energy ball milling wascarried out in a Union Process model HDMM using 625 micron zirconiumdioxide (ZrO₂) balls. The HEBM process was performed in ethanol media inair where the machine was operated at 3000 rpm for one hour with apowder to balls ratio of 1:20

Later on, the mixtures were oven dried at 80° C. for 12 h to evaporatethe ethanol. The aforesaid sonicated powder mixtures were sintered into20 mm discs at 1500° C. with a holding time of 30 minutes, whileemploying a constant pressure of 50 MPa. The heating rate use for thesynthesis process was 100° C./min. Samples were then rapidly cooled downto ambient temperature within a timeframe of 10 min.

The sintered samples were then cleaned to remove graphite, mounted,ground, and polished using diamond discs according to standardmetallography procedure. For identification of phases present in thesynthesized samples, Rigaku MiniFlex X-ray diffractometer (Japan) wasused with Cu K_(α1) radiation (γ=0.15416 nm), a tube current of 10 mA,and an accelerating voltage of 30 kV. Microstructural studies wereperformed using a field emission scanning electron microscope (FESEM,Lyra 3, Tescan, Czech Republic) with accelerating voltages of 20-30 kV.Archimedes' principle was applied to evaluate the density of thesintered samples. A universal hardness tester (Zwick-Roell, ZHU250,Germany) was employed to evaluate the Vickers hardness of sinteredsamples under a load of 100 N. The fracture toughness was evaluatedbased on the indentation method.

TABLE 1 Classification of composites as per the particle size,reinforcement (cBN) weight percent, and AlN particle size. cBN ParticleAlN Precursor Sample ID Size cBN Wt. % Size (nm) Coating 5-Ca-α-1 — — —No 5-Ca-α-2 20 μm 10% 100 No 5-Ca-α-3 20 μm 20% 100 No 5-Ca-α-4 20 μm30% 100 No 5-Ca-α-5 40 μm 10% 50 No 5-Ca-α-6 40 μm 20% 50 No 5-Ca-α-7 40μm 30% 50 No 5-Ca-α-8 40 μm 10% 50 Nickel Coated cBN 5-Ca-α-9 40 μm 20%50 Nickel Coated cBN 5-Ca-α-10 40 μm 30% 50 Nickel Coated cBN

EXAMPLE 2 Results and Discussion—Phase Assemblage

X-ray diffraction patterns of the sintered samples 5-Ca-α-2, 5-Ca-α-3,and 5-Ca-α-4 each having 10, 20, and 30 weight percent of 20 μm cBN,respectively, are shown in FIG. 1. These samples were sintered at 1500°C. and a holding time of 30 min. Sialon and cBN (20 μm) were matchedwith ICDD #00-042-0252 and 01-089-1498, respectively. For the samplessynthesized with the aid of 40 μm cBN reinforcement (5-Ca-α-5 to5-Ca-α-10), diffraction patterns are shown in FIG. 2. These samples werealso sintered at 1500° C. and with a holding time of 30 min. Sialon andcBN (40 μm) were matched with ICDD #00-048-1615 (β-Si₁₀Al₂O₂N₁₄),01-089-1498 (cBN), and 01-07708869 (hBN—hexagonal boron nitride).

Composites reinforced with 20 μm cBN showed the presence of pureCa-alpha-sialon (Ca_(0.68)Si_(9.96)Al_(2.04)O_(0.68)N_(15.32)) phase asper the designed values of m and n. Aside from the pure calciumstabilized alpha sialon phase, no evidence of cBN to hBN transition wasrecorded by the X-ray diffraction. However, for the 50 nm AlN samples(5-Ca-α-5 to 5-Ca-α-10), a complete alpha to beta transformation wasrecorded for the sialon matrix. These samples also showed a major cBN tohBN transformation as evident from the XRD pattern. The transition fromcBN to hBN for the said composites was confirmed with the help of Ramanmicroscopy, as shown in FIG. 3. This figure shows a post-sintering Ramanmicroscopy scan of a 40 μm cBN particle embedded in a sialon matrix(sample 5-Ca-α-7), depicting the high intensity peak characteristic ofhBN. While the exact transformation mechanism of cBN to hBN is stillunder investigation, it is likely that during the sintering at 1500° C.,the smaller particle size of the AlN precursor facilitates thetransition of the stable alpha sialon phase into the beta sialon phase.

EXAMPLE 3 Results and Discussion—Microstructure Development

A FESEM micrograph of an etched pure alpha sialon sample (5-Ca-α-1) isshown in FIG. 4A. Full dense single phase equiaxed alpha-sialon grainscan be observed in this micrograph, which validates the existence of thesingle phase alpha sialon by choosing an amount of precursors accordingto the formula values of m=1.6 and n=1.2.

A typical backscattered FESEM micrograph of the composite containing 10wt % of 20 μm cBN reinforcement (5-Ca-α-2) is shown in FIG. 4B.According to this micrograph, cBN and alpha-sialon can be distinguishedeasily due to their different mean atomic number. The cBN grains (inblack contrast) are homogeneously dispersed in continuous alpha-sialonmatrix, and no pore was found in all the samples.

Microstructural images of sialon composites reinforced with 30 wt % 40μm cBN (5-Ca-α-7) are shown in FIGS. 5A and 5B. The uniform distributionof reinforcement particles within the matrix may be easily observed.FIG. 5A shows a polished surface of the sample, while an etched surfacethe same is depicted in FIG. 5B. The needle shaped, elongated morphologyof the grains in FIG. 5B are evidence of the alpha to betatransformation of the sialon matrix. While the transformation mechanismfrom alpha to beta at these conditions may not be completely understood,it may be attributed to the creation of a larger amount of oxygen-richliquids due to the smaller particle size of the aluminum nitrideprecursors having a higher reactivity.

EXAMPLE 3 Results and Discussion—Role of cBN Particle Size and AlNParticle Size on Densification of Composites

The graph in FIG. 6 depicts the trends in densities that were observedas a function of reinforcement weight percentage. Pure alpha sialonsample (5-Ca-α-1) displayed the highest density of 3.5 g/cm³. Increasingthe weight percent of reinforcement lead to a slight decrease indensity, where the 5-Ca-α-4 (30 wt % of 20 μm cBN) displayed a densityof 3.31 g/cm³, which represents a densification of about 95% as comparedto the pure alpha sialon. This negative effect of cBN on thedensification has also been reported in cBN-sialon, cBN—Al₂O₃ and,cBN—WC—Co composites [Feng Y. et al. Mater. Sci. Eng. A 527 (2010)4723-6; Hotta, M. et al. J. Ceram. Soc. Jpn. 116 (2008) 744-8; andMartinez, V. et al. J. Am. Ceram. Soc. 90 (2007) 415-24 eachincorporated herein by reference in its entirety]. The heating nature ofSPS, i.e. pulsed-current, aids the densification process at lowsintering temperature, along with the higher surface areas associatedwith the nano precursors. CaO, as a densifying additive, provided anextra amount of liquid phase at the first stages of sintering, whichfacilitated densification. A large surface area associated with nanopowder has been proven to act as the driving mechanism for sintering togain thermal equilibrium [Groza, J. R. Nanostruct. Mater. 12 (1999)987-92—incorporated herein by reference in its entirety]. The equationdeveloped by Hansen and his group estimates that the sintering ratewould be enhanced by a factor of 4 if the starting powder material sizeis reduced by one order of magnitude [Hansen, J. D. et al. J. Am. Ceram.Soc. 75 (1992) 1129-35—incorporated herein by reference in itsentirety].

However, a decline in densities of alpha-sialon composites reinforcedwith 40 μm cBN is most likely due to the larger size of filler particlesimpeding the densification process. Moreover, an unusual alpha to betaphase transformation observed in the high energy ball milled samples(5-Ca-α-5 to 5-Ca-α-10) may have hindered the densification process dueto the simultaneous structural changes taking place during sintering.

EXAMPLE 4 Results and Discussion—Role of cBN Particle Size and AlNParticle Size on Mechanical Characteristics

Table 2 summarizes the results of mechanical testing for the pure sialonas well as sialon reinforced cBN composites. For the samples preparedvia sonication route (5-Ca-α-1 to 5-Ca-α-4), addition of cBN strengthensthe matrix as indicated by the increase in Vickers hardness values. Withincreasing the weight percent of 20 μm cBN to 30 wt %, the Vickershardness increases up to 24.01 GPa, as compared to the hardness of 21.06GPa for the pure alpha sialon matrix. However, while increasing theweight percent of 20 μm cBN, the fracture toughness was found todecrease.

The composites prepared by employing AlN of 50 nm particle size(5-Ca-α-5 to 5-Ca-α-10) showed a decrease in hardness values uponaddition of 40 μm cBN when compared to the pure alpha-sialon matrix.This decrease in hardness values results from the unanticipated phasetransformations of both alpha to beta sialon as well as cBN to hBN.However, an increase in fracture toughness values for the said samplesis a typical result of the elongated morphology of beta-sialon grains.Composites reinforced with nickel-coated cBN particles (5-Ca-α-8 to5-Ca-α-10) display a more promising fracture toughness due to a muchsmoother interface between the reinforcement and the matrix.

Cubic boron nitride (cBN) reinforced sialon composites prepared viaprobe sonication, were sintered by SPS at 1500° C. for 30 minutes undera constant pressure of 50 MPa. The cBN particles could be evenlydispersed in the sialon matrix with this ultra-sonication technique.Using a smaller size of precursor particles resulted in an unanticipatedphase transformation from alpha to beta sialon. Particles of cBN (20 μm)may be embedded in the alpha sialon matrix as a way to producecomposites achieving hardness values as high as 24.01 GPa with amoderate fracture toughness of about 5.67 MPa√m. The transformationmechanism from alpha to beta sialon, triggered by the smaller size ofaluminum nitride precursor, is being investigated.

TABLE 2 Mechanical Properties of the Samples. Hardness - HV₁₀ FractureToughness Sample ID (GPa) (MPa√m) 5-Ca-α-1 21.06 7.29 5-Ca-α-2 21.849.03 5-Ca-α-3 23.10 6.70 5-Ca-α-4 24.01 5.67 5-Ca-α-5 15.5 7.95 5-Ca-α-611.6 11.51 5-Ca-α-7 9.4 12.46 5-Ca-α-8 13.5 10.35 5-Ca-α-9 11.2 11.575-Ca-α-10 8.3 12.97

1: A method for producing a composite of cubic boron nitride (cBN)dispersed in a SiAlON ceramic, the method comprising: mixing siliconnitride nanoparticles, aluminum nitride nanoparticles, silicananoparticles, calcium oxide nanoparticles, and cubic boron nitride(cBN) microparticles to produce a mixture, and sintering the mixture toproduce the composite. 2: The method of claim 1, wherein the mixinginvolves sonication. 3: The method of claim 1, wherein the mixinginvolves ball milling. 4: The method of claim 1, wherein the cBNmicroparticles have a largest linear dimension of 10-50 μm and arepresent in the mixture at a weight percentage of 5-40 wt %, relative toa total weight of the mixture. 5: The method of claim 4, wherein themixture comprises nickel, the nickel located on an exterior surface ofthe cBN microparticles. 6: The method of claim 5, wherein the cBNmicroparticles are coated with nickel and comprise 20-80 wt % nickel,based on a total weight of the cBN microparticles. 7: The method ofclaim 6, wherein the composite has a higher fracture toughness than anotherwise identical composite sintered from cBN microparticles that donot have nickel. 8: The method of claim 1, wherein the silicon nitridenanoparticles comprise α-Si₃N₄. 9: The method of claim 1, wherein thealuminum nitride nanoparticles have a longest linear dimension of 30-120nm. 10: The method of claim 1, wherein the aluminum nitridenanoparticles have a longest linear dimension of 30-70 nm. 11: Themethod of claim 10, wherein the composite has a higher fracturetoughness than an otherwise identical composite produced from otheraluminum nitride nanoparticles having a longest linear dimension of85-500 nm. 12: The method of claim 10, wherein at least 75 wt % of theSiAlON ceramic is in a β phase, relative to a total weight of the SiAlONceramic. 13: The method of claim 10, wherein the composite comprisesboron nitride, and 40-95 wt % of the boron nitride relative to a totalweight of the boron nitride is hexagonal boron nitride (hBN) asdetermined by XRD and/or Raman spectroscopy. 14: The method of claim 1,wherein the sintering is a spark plasma sintering process. 15: Themethod of claim 1, wherein the sintering is performed at a temperatureranging from 1400-1600° C. 16: The method of claim 1, wherein thesintering comprises heating the mixture at a rate ranging from 5-600°C./min. 17: The method of claim 1, wherein the sintering comprisesheating the mixture at a rate ranging from 90-110° C./min. 18: Themethod of claim 1, wherein the sintering comprises applying a uniaxialpressure ranging from 30-80 MPa to the mixture. 19: The method of claim1, wherein the composite has a Vickers hardness (HV₁₀) of 8-25 GPa. 20:The method of claim 1, wherein the composite has a fracture toughness of5-13 MPa√m.